Ferritic steels with improved corrosion resistance, such as AISI 409 which contains 0.03%C max, 10.5-11.7%Cr and titanium as a stabilizer, although frequently used for structural applications, have a number of disadvantages including their relatively low toughness and yield strength. In addition they may not be suitable for certain structural applications in which welding is required.
To overcome some of the limitations of ferritic steels such as AISI 409, steels with a ferritic-martensitic microstructure and improved strength and weldability have been developed. Typical of these steels is 3CR12 which nominally contains 0.025% C, 11.45% Cr, 0.4% Ni, 0.8%max Mn, 0.45%max Si, 0.21% Ti, 0.014% S, 0.020% P and 0.013% N and is further described in South African Pat. No. 78/4764. The titanium addition is for the purpose of modifying the response of the steel to heat treatment. The residual molybdenum content in these steels is about 0.04%.
Steels such as 3CR12 may not have sufficient impact strength for certain applications. In addition the titanium may contribute to the formation of a coarse grained ferritic structure in the heat affected zone upon welding. A coarse grained ferritic weld zone will have reduced toughness. Thus the need exists for a steel which can be welded while maintaining strength and toughness.